Valves for controlling the flow of liquids and gases often employ main seal rings of resilient materials in order to effect a more fluid-tight seal. However, when such valves are used in pipelines or other fluid flow systems which conduct combustible materials, such seal rings are subject to destruction in the event of fire, when they are needed most from the standpoint of safety. Accordingly, it is highly desirable to prevent flow of the fluid to aid or foster combustion and to keep it confined, in the event of failure of the principal seal.